Active sound cancellation or active noise control (ANC) is known as noise countermeasures against a rotation sound source caused by rotor blades or a rotation sound source caused by interference of rotor blades and stator blades. The active noise control is a technique of reducing noise by causing a speaker to output control sound that has the same amplitude and the reverse phase relative to the noise.
However, the active noise control against a continuous rotation sound source simulates the rotation sound source by speakers arranged in a discrete manner and thus requires many speakers. This increases the total weight of the active noise reduction apparatus including the speakers and requires a space for installation, which makes it difficult to mount the apparatus on, for example, a jet engine, etc.